The invention relates to a decoupler for the rotational drive of a generator of an auxiliary unit belt drive of an internal combustion engine, with:                a belt pulley rotationally driven by the belt,        a hub that rotationally drives the shaft of the generator and is arranged radially inside the belt pulley,        two axially spaced bearing points at which the belt pulley is supported so that it can rotate on the hub,        a series arrangement radially between the belt pulley and the hub formed of a helical torsion spring and a one-way coupling that allows the hub to overtake the belt pulley in the driving rotational direction,        a spring plate that is rotationally fixed relative to the belt pulley or the hub for the one end of the helical torsion spring and a spring plate that can rotate relative to the belt pulley and the hub for the other end of the helical torsion spring,        
The spring plates each rise up to a step axially with a ramp-like shape and the spring ends contacting the ramp steps expand the helical torsion spring radially while transferring the drive moment. Here, the friction moment generated in one of the bearing points by the overrunning hub loads the rotating spring plate in the rotational direction of the rotationally fixed spring plate.
Such decouplers compensate for known rotational oscillations and non-uniformity of the crankshaft transmitted via the auxiliary unit belt drive to the generator. The series arrangement made from the one-way coupling and the helical torsion spring transfers, in the closed state of the one-way coupling, the drive moment of the belt via the belt pulley and the hub to the shaft of the generator, wherein the elasticity of the helical torsion spring smooths the rotational non-uniformity. For a delayed rotating belt pulley, the one-way coupling opens, wherein—then inversely—no significant torque can be transferred from the hub to the belt pulley and the generator shaft with relatively large mass inertia overtakes the belt pulley.
The location of this decoupling effect can also be directly on the crankshaft as is generally known, wherein then the helical torsion spring and the one-way coupling connected in series, for appropriate dimensioning and matching to the reversed flow of drive torque, are part of a so-called crankshaft decoupler that is positioned on the crankshaft and drives the entire auxiliary unit belt drive.
A generator decoupler according to the class is disclosed, for example, in EP 2 258 968 A1. The one-way coupling is a clamping body overriding clutch that is arranged downstream of the belt pulley on the hub with respect to the flow of drive torque, i.e., behind the helical torsion spring and is consequently arranged directly on the hub. The rotating bearing of the belt pulley is realized on the generator side by a ball bearing and opposite this, on one hand, by a sliding bearing ring in which the rotating spring plate is supported and, on the other hand, by another ball bearing that supports the rotating spring plate on the hub.
Although the one-way coupling is open for an overrunning hub, the unavoidable residual friction moment of the coupling can lead to a relative rotation of the two spring plates, wherein one or two ends of the helical torsion spring are at a distance from the surrounding ramp steps of the spring plates and run upward at their ramps. The effectively decreasing axial installation space due to the ramp geometry can have the effect for the helical torsion spring that the helical torsion spring presses the two spring plates axially apart from each other and thus the decoupler more or less jumps axially. A similarly undesired consequence is the noticeable acoustics of the decoupler when one or both spring ends repeatedly run up the ramps and snap back to the ramp steps after each rotation.
This problem in the ramp run-up is corrected in a decoupler as known from U.S. Pat. No. 8,047,920 B2 by a mechanism that prevents relative rotation of the two spring plates by opposing stops for an overrunning hub. Such a mechanism can be required, in particular, if the one-way coupling—as in this publication—is a looped belt whose residual friction in the opened state is naturally so large it forces the ramp run-up.